[cfsmp3]

In raising this pull request, I confirm the following (please check boxes):

Reason for this PR:

• This PR adds new functionality.
• This PR fixes a bug that I have personally experienced or that a real user has reported and for which a sample exists.
• This PR is porting code from C to Rust.

Sanity check:

• I have read and understood the contributors guide.
• I have checked that another pull request for this purpose does not exist.
• If the PR adds new functionality, I've added it to the changelog. If it's just a bug fix, I have NOT added it to the changelog.
• I am NOT adding new C code unless it's to fix an existing, reproducible bug.

Repro instructions:

Build with ./build -debug and run ccextractor b8c55aa2e9f27f1355c3b7848e513474a78d1efb56973134fd9f665e6c86a2ef.mpg --tpages-all -o out.srt (teletext sample). It aborts:

unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null
(panicked at src/libccxr_exports/mod.rs:105:14)

Release builds don't check the precondition, so there it is silent undefined behaviour.

---

C represents an empty compare buffer as (NULL, 0): telxcc.c resets ucs2_buffer_cur to NULL after every buffer switch, and process_page() → fuzzy_memcmp() passes it to levenshtein_dist() when a page carried no text. That's fine in C, but slice::from_raw_parts requires a non-null pointer even for a zero length, so ccxr_levenshtein_dist() was UB.

The slices are now built through a helper that maps NULL to an empty slice; ccxr_levenshtein_dist_char() uses it too, since it has the same hazard. The Safety docs say NULL denotes an empty slice.

Verified:

• New unit tests call both exports with null pointers / zero lengths. Without the fix the test binary aborts on the same precondition; with it they pass.
• The debug build no longer aborts on the sample above (all 9 page files written).
• 108 teletext runs (platform commands, --teletext, --tpages-all srt/webvtt across 29 samples): release output and exit codes byte-identical to master.
• cargo test, cargo fmt --check and cargo clippy -D warnings clean in both crates.